An automatic implantable cardioverter defibrillator (AICD) or implantable cardioverter defibrillator (ICD) detects ventricular fibrillation and delivers a series of countershocks of sufficient energy to terminate the fibrillation. Such an ICD utilizes an electrode system either attached to the outer surface of the heart by means of a large surface area patch electrode, or inserted transvenously into or near the heart. Such an ICD system may be combined with a pacemaker function.
Transvenous defibrillator leads for correcting ventricular tachycardia and ventricular fibrillation include uninsulated, helically wound shocking electrodes, formed of round wire, and rely on direct contact between the electrode and tissue or blood within or near the heart to deliver electrical energy to the heart.
The shocking coil of a chronically implanted, transvenous defibrillator lead poses two significant, yet different, problems. First, because the surface of a helical shocking coil is contoured and has a large area, it tends to encourage tissue ingrowth, that is, the growth of tissue into the interstices of the exposed coil windings. This tissue ingrowth can make removal of the lead difficult and in the event of post-op infection can present a significant risk to the patient in the event removal of the lead is required because of an uncorrectable device malfunction. Extensive surgical intervention may be required with a concomitant lengthy recovery time. Secondly, transvenous defibrillator leads with shocking electrodes formed from round wire and inserted within the superior vena cavae have a tendency to encourage the attachment of red blood cells and platelets (thrombus). More specifically, the high energy densities can produce instantaneous temperatures high enough to denature surface proteins, damage platelets and alter cell membrane potentials to such an extent as to initiate thrombosis. A lack of hemocompatibility as described can result in emboli which pose serious risks, the accumulation of thrombus at the shocking electrode being potentially damaging to the patient since these can lead to a decrease in blood flow, infarct or stroke.
It is known to coat or otherwise cover a helically wound transvenous defibrillator electrode with an electrically conductive polymeric material for inhibiting tissue ingrowth, thus reducing the risk to the patient in the event removal of the lead becomes necessary. One such material is PTFE, a porous, biocompatible insulating material that becomes conductive as body fluids penetrate the pores. The small pore sizes, however, tend to inhibit tissue ingrowth.
U.S. Pat. No. 5,090,422 discloses an ICD lead including an endocardial helically wound electrode covered with a thin coating membrane of biocompatible porous material relatively inert to bodily fluids. The material is preferably constructed of a polyurethane foam or other biocompatible, relatively soft polymeric material that can be produced within the desired range of pore sizes. Particular materials that are disclosed in the '422 patent include woven, porous polyurethane and porous polytetrafluoroethylene (PTFE) used with a wetting agent or with a modified surface. The '422 patent proposes materials having an average surface pore size less than about 15 to 20 microns to ensure a dissection plane which precludes significant tissue ingrowth yet allows the penetration or passage of bodily fluids thereby reducing the electrical resistance and minimizing ohmic losses in the system.
U.S. Pat. No. 4,573,480 relates to an implantable cardiac pacemaker lead covered with an insulating sheath made of porous PTFE instead of the more commonly used silicone rubber or polyurethane. The sizes of the pores are selected to combine high flexibility with sufficient imperviousness to body fluid. The insulation can have areas of varying porosity along its length. For example, in the vicinity of the electrodes, areas with large pores serve to ensure that this part of the electrode will grow in after the implantation.